Actuating circuit for D.C. motor

ABSTRACT

A D.C. motor control circuit including a wheatstone bridge type speed regulating circuit with a transistor arranged for detection of the counter-EMF developed across the motor armature to control operation of a drive control circuit. In one embodiment of the invention, an actuating circuit comprises a constant voltage source formed from a constant current circuit connected parallel to an electrical power supply source for the motor and a variable resistor connected in series to the constant current voltage source. A diode applies the output of the variable resistor to the base of the transistor to thereby effect conduction of this transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to D.C. motor speed control apparatus, and moreparticularly to a D.C. motor speed control apparatus of the type inwhich the armature winding of the motor is connected in one branch of aWheatstone bridge circuit with an output proportional to the shaft speedbeing utilized in controlling the electrical power supply to the motor.Still more particularly, it relates to improvements of an actuatingcircuit associated with such a motor speed control circuit.

2. Description of the Prior Art

A D.C. motor speed control circuit of the Wheatstone bridge type is, forexample, disclosed in Japanese Patent Publication No.Sho 50-40442 (Dateof Publication: Dec. 24, 1975). FIG. 1 shows an example of this type ofcircuit. Here a D.C. motor DCM with an armature winding M having aresistance Ra forms a Wheatstone bridge together with three resistorsR₁, R₂ and R₃. Connected across a pair of output terminals or points aand b, is an npn transistor Tr₂ and a spring of two diodes D₁ and D₂.The base of transistor TR₂ is connected to the point a, betweenregistors R₂ and R₃. Its emitter is connected through a resistor R₄ to anegative input terminal or point d between resistor R₃ and the motorarmature M. The emitter is also reverse biased by the string of twodiodes D₁ from the point b between resistor R₁ and the motor armature.Hence the collector current depending upon the counter-EMF developedacross the armature can be utilized to control the electrical powersupply to the motor by using a pnp transistor Tr₁. The latter isconnected between a positive input terminal c of the Wheatstone bridgecircuit and a positive electrode of an electrical power source orbattery E through a control switch S₁.

The voltage Et across the motor armature M is composed of twocomponents; one is an equivalent developed counter electromotive force(EMF) Ea and the second is a voltage across the armature resistance Ra,and therefore may be expressed as

    Et=EA+RaIa                                                 (1)

Here Ia is the current flowing through the armature resistance Ra. Whenthe resistance Ra is selected so that the relationship Ra/R₁ =R₃ /R₂ issatisfied, the voltage appearing across the output terminals a and b ofthe Wheatstone bridge circuit is equal to the counter-EMF developedacross the motor armature M. This counter-EMF Ea may be expressed as

    Ea=(Z/a)·p·φ·n              (2)

wherein Z is the number of conductors; p is the number of poles; φ isthe magnetic flux density per pole; a is the number of turns of wire;and n is the number of revolutions of the rotor. All of the aboveparameters except the n are constants of the design, thus the equation(2) may be written,

    Ea=Kn                                                      (3)

With this Wheatstone bridge circuit, therefore, it is possible tocontrol the number of revolutions of the rotor, that is, the speed ofrotation of the motor, as a function of only one variable Ea in a mannersuch that the output of the Wheatstone bridge circuit is applied to thebase of the transistor Tr₁. This can then control the current flowingthrough the emitter-collector of transistor Tr₁.

In operation, after a stationary state has been attained to drive themotor DCM at a constant speed, the output voltage V_(D) of theWheatstone bridge circuit is maintained constant depending upon thedifference between the voltage V_(F) across diodes D₁ and thebase-emitter voltage V_(EB) of transistor Tr₂ regardless of how muchcurrent flows through the diodes D₁ and the transistor Tr₂. Assumingthat load on the motor DCM is increased a decrease occurs in the numberof revolutions of the rotor. The voltage Et across the motor winding isthen decreased. This decreases the potential at the point b, along withthe emitter potential of transistor Tr₂. This in turn causes an increasein the base potential of transistor Tr₁ relative to the emitterpotential. As a result, the current supply to the Wheatstone bridgecircuit is increased until the speed of rotation of the motor isrestored to the initial predetermined level.

The conventional actuating circuit for use with the D.C. motor speedcontrol circuit of FIG. 1 is shown in the same figure. It includes aresistor R₅ and a capacitor C₁. These are connected in series betweenthe base electrode of transistor Tr₁ and the negative terminal of thedirect current supply source E. This actuating circuit is, however, notsuited for assurance of initiation of operation of the speed controlcircuit at the time of closure of the main switch S₁. For example, whenthe voltage of the energy source E increases at a slow rate to thecritical level for the normal operation of the motor DCM, a long timelag results. Alternatively when switch S₁ is opened at the time themotor DCM is restrained, the charge stored on the capacitor C₁ isretained except for spontaneous leakage because of the lack of thedischarge circuit therefor. This is so because the transistor Tr₂ is cutoff at that time. Consequently, the subsequent closure of switch S₁before capacitor C₁ is not completely discharged fails to result inactuation of the motor DCM, for transistor Tr₁ is not renderedconductive between the emitter and base thereof.

SUMMARY OF THE INVENTION

The present invention concerns the above mentioned Wheatstone bridgetype of D.C. motor speed control apparatus. It is an object of theinvention to improve the actuating circuit and to assure initiation ofoperation of the D.C. motor any time it is activated.

Another object is to provide an improved actuating circuit associatedwith the speed control circuit without adversely affecting the speedcontrol operation.

Still another object is to reduce the consumption of electrical energywhich would otherwise result from use of the actuating circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a conventional D.C. motor controlapparatus.

FIG. 2 is a circuit diagram of one embodiment of a D.C. motor controlapparatus according to the present invention.

FIG. 3 is a similar diagram showing a second embodiment of theinvention.

FIG. 4 is a similar diagram showing a third embodiment of the invention.

FIG. 5 is a similar diagram showing a fourth embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a D.C. motor control circuit embodying the invention. Herethe right-hand portion of the circuit is a Wheatstone type speed controlcircuit of conventional structure similar to that of FIG. 1. The samereference characters have been employed to denote parts similar to thoseof FIG. 1. The left-hand portion is an actuating circuit usingcharacteristics of the present invention. In the actuating circuit ofFIG. 2 a pnp transistor Tr₃ has an emitter electrode connected through aresistor R₆ to the positive bus at a point between the main switch S₁and the emitter of the power supply control transistor Tr₁. A baseelectrode of the transistor Tr₃ is connected to a junction between adiode D₂ and a resistor R₇ which are connected in series across thepositive and negative buses. The parts Tr₃, R₆, R₇ and D₂ form aconstant current circuit. A variable resistor VR₁ in the actuatingcircuit is connected between the collector of transistor Tr₃ and thenegative bus. The output of the variable resistor VR₁ appears at a pointe between the collector of transistor Tr₃ and the variable resistor VR₁.This point is connected to the output a of the Wheatstone bridgecircuit, and the base electrode of the transistor Tr₂ through aunidirectional conductor or diode D₃ poled toward the latter.

The operation of the apparatus of FIG. 2 is as follows. When the powerswitch S₁ is closed, a current of predetermined level flows through thevariable resistor VR₁ from the constant current circuit. This renderstransistor Tr₂ conductive if the variable resistor VR₁ is adjusted toproduce an output voltage higher than the threshold level of thetransistor Tr₂. When the transistor Tr₂ conducts, the transistor Tr₁operates to supply power to the Wheatstone bridge circuit andsimultaneously acutuate the D.C. motor DCM.

An advantage of the actuating circuit of the invention derives from theuse of the constant current circuit, Tr₃, R₆, R₇, D₂. Because of thiscircuit even when the electrical power source E operates along a slowvoltage ramp, the output voltage of the variable resistor VR₁ iscoincident with the predetermined level therefor. This assuresinitiation of the motor DCM at the time the main switch S₁ is closed.

Another advantage is that the the invention permits opening of switch S₁at the time the motor DCM is restrained, though this renders thetransistors Tr₁, Tr₂ and Tr₃ non-conducting as in the prior artapparatus of FIG. 1. This is possible because when switch S₁ is thrownagain, transistor T₂ is forcibly turned on by the voltage generatedacross the variable resistor VR₁ so that the motor DCM is actuatedwithout fail.

A further advantage is that after the actuation of the motor DCM hasbeen initiated, the potential at the point a becomes higher than that atthe point. Hence no current flows from the actuating circuit to theWheatstone bridge circuit. This cuts the actuating circuit off from itsconnection with the latter. During the operation of the motor DCM,therefore, the actuating circuit produces no adverse effect on the speedcontrol of the motor DCM. This is true even when the voltage of thesupply source E is permitted to increase to a considerable level afterthe actuation of the motor DCM, as the voltage across the variableresistor VR₁ is maintained constant by the constant current circuit Tr₃,R₆, R₇, D₂. Also, the one-way conductive means or diode D₃ functions toprevent occurrence of a forward current flow from the actuating circuitto the speed control circuit after the production of the voltage atpoint a higher than that at point e.

The use of the variable resistor VR₁ in the actuating circuit providesan advantage. Assume that the potential at the point e is made slightlyhigher than the threshold level of the transistor Tr₂ and that thedesired speed of the motor DCM is made very high then the differencebetween the potential at point a corresponding to the selected speed ofthe motor DCM and the potential at point e becomes quite large. Even inthis case, it is of course possible to effect actuation of motor DCM,though a considerable time is required for the actual speed of motor DCMto reach the desired speed. This is so because the transistor Tr₂ isdriven only by the current flowing through the resistor R₂. Theactuating current flowing through diode D₃ is stopped as soon as thetransistor Tr₂ is rendered conductive. In order to reduce this time lag,however, according to the invention, the operator need only to adjustthe variable resistor VR₁ so that the potential at point e becomesalmost equal to that at point a which corresponds to the desired speedof motor DCM. If so, the actuating circuit continues to supply thevoltage to the transistor Tr₂ until a time just before the motor DCMreaches the desired speed.

In order to remove electrical noise and to prevent oscillation, thereare provided two capacitors C₂ and C₃ respectively connected between thebase and collector of transistor Tr₂ and between the base of transistorTr₂ and the negative bus.

In FIG. 3, there is shown another embodiment of the actuating circuitaccording to the invention. Here the constant current circuit isconstructed with a field effect transistor Tr₄ arranged with its drainelectrode connected through a resistor R₈ to the positive bus. Itssource electrode is connected through a dynamiccharacteristics-adjusting variable resistor VR₂ to a variable resistorVR₃ corresponding to that VR₁ of FIG. 2. Its substrate electrode isconnected both to the junction between the variable resistors VR₂ andVR₃ and to the anode of diode D₃. Upon proper adjustment of the variableresistor VR₂, the transistor TR₄ is set to an optimum operating positionas it serves as a constant current source.

In the above two embodiments of FIGS. 2 and 3, there is a disadvantagethat the actuating current exiting from the diode D₃ is applied not onlyto the transistor Tr₂ but also to the resistor R₃ constituting onebranch of the Wheatstone bridge circuit. That portion of the actuatingcurrent which flows through the resistor R₃ is entirely wasteful as nouse is made in actuating the motor DCM. In order to eliminate thisdisadvantage, the present invention contemplates to use an additionaldiode D₄ for connection between the diode D₃ and the resistor R₃.

FIGS. 4 and 5 show additional two embodiments which differ respectivelyfrom those of FIGS. 2 and 3 in using this diode D₄ as arranged with itsanode connected to the output point, a, of the Wheatstone bridge circuitand with its cathode connected to the base electrode of transistor Tr₂.The cathode of diode D₃ is connected to the junction between the diodeD₄ and the base electrode of transistor Tr₂.

It will be seen from the foregoing that the present invention provides aD.C. motor control apparatus which enables the D.C. motor to be actuatedfor rotation without delay from the time when an actuating switch isoperated. Even when the voltage of the power supply source fallsconsiderably below a satisfactory operating level for the motor, theactuation of the motor can be secured. Further the apparatus of theinvention permits the motor to be accelerated along a high rampregardless of how fast a speed is selected, as the variable resistor VR₁or VR₃ is adjusted to the corresponding resistance value, and then to beoperated with minimum energy consumption.

What is claimed is:
 1. A speed control sevice for D.C. motorcomprising:(a) an electric power supply circuit; (b) a bridge circuithaving branches and output as well as input terminals, the bridgecircuit including the D.C. motor in one branch thereof and producing atits output terminals a first voltage corresponding to the speed ofrotation of the D.C. motor; (c) driving means connected between theelectric power supply circuit and the bridge circuit to supply drivingcurrent to said bridge circuit; (d) driving current control meanscoupled to said driving means and said output terminals for controllingsaid driving means on the basis of the first voltgage; (e) a controlcircuit including a constant current circuit and voltage preset meanscoupled to the constant current source for forming a constant voltagecurrent, said control circuit being connected to said electric powersupply circuit; and (f) one-way conductive means connected between theoutput of said constant voltage circuit and said driving current controlmeans to apply an energizing voltage from said constant voltage circuitto the input terminal of said driving current control means at the timeof actuation of said D.C. motor.
 2. A speed control device according toclaim 1, wherein said voltage preset means includes an impedance elementconnected in series to said constant current circuit.
 3. A speed controldevice according to claim 2, wherein said impedance element consists ofa resistor.
 4. A speed control device for D.C. motor comprising:(a) anelectric power supply circuit; (b) a resistor bridge circuit havingbranches and output terminals, the resistor bridge circuit including theD.C. motor in one branch thereof and producing from the output terminalsa first voltage corresponding to the speed of rotation of the D.C.motor; (c) driving means connected between the electric power supplycircuit and the resistor bridge circuit to supply driving current tosaid resistor bridge circuit; (d) driving current control means coupledto at least an output terminal of the bridge circuit for controllingsaid driving means on the basis of said first voltage; (e) a constantvoltage circuit comprising a constant current circuit and variableresistor means connected in series with said constant current circuitfor producing a desired output voltage, said constant voltage circuitbeing connected parallel to said electric power supply circuit; saidconstant voltage circuit having an output terminal and said drivingcurrent control means having an input terminal; and (f) one-wayconductive means connected between the output terminal of said constantvoltage circuit and the input terminal of said driving current controlmeans to apply an energizing voltage from said constant voltage circuitto the input terminal of said driving current control means at the timeof actuation of said D.C. motor.
 5. A speed control device for D.C.motor, comprising:(a) an electric power supply circuit; (b) a resistorbridge circuit having branches and output terminals, the resistor bridgecircuit including the D.C. motor in one branch thereof and producingfrom the output terminal a first voltage corresponding to the speed ofrotation of the D.C. motor; (c) driving means connected between theelectric power supply circuit and the resistor bridge circuit to supplydriving current to said resistor bridge circuit; (d) driving currentcontrol means for controlling said driving means in accordance with thedifference between said first voltage and a reference voltage; (e) aconstant voltage circuit including a constant current circuit, saidconstant voltage circuit being connected parallel to said electric powersupplying circuit; (f) first one-way conductive means connected betweenthe output terminal of said constant voltage circuit and the inputterminal of said driving current control means in order to apply theoutput voltage of said constant voltage circuit to the input terminal ofsaid driving current control means at the time of actuation of said D.C.motor; and (g) second one-way conductive means connected between theoutput terminal of said first one-way conductive means and anintermediate point of resistor bridges of said resistor bridge circuitso as to hinder output voltage from said constant voltage circuit.
 6. Aspeed control device according to claim 5, wherein said second one-wayconductive means consists of a diode.
 7. A speed control device for aD.C. motor, comprising:(a) an electric power supply circuit; (b) aresistor bridge having branches and output terminals, the resistorbridge including the D.C. motor in one branch thereof and producing fromthe output terminals a first voltage corresponding to the speed ofrotation of the D.C. motor; (c) driving means connected between theelectric power supply circuit and the resistor bridge to supply drivingcurrent to said resistor bridge; (d) driving current control means forcontrolling said driving means on the basis of said first voltage; (e) acontrol circuit including a constant current circuit and voltage presetmeans coupled to the constant current circuit for forming a constantvoltage circuit, said control circuit being connected to said electricpower circuit; said constant voltage circuit having an output terminaland said driving current control means having an input terminal; (f)first one-way conductive means connected between the output terminal ofsaid constant voltage circuit and the input terminal of said drivingcurrent control means to apply an energizing voltage from said constantvoltage circuit to the input terminal of said driving current controlmeans at the time of actuation of the D.C. motor; and (g) second one-wayconductive means connected between the output terminal of said firstone-way conductive means and at a point between the branches of saidresistor bridge so as to restrict the output voltage from said constantvoltage circuit.